As long as there have been tacky polymers or blends of polymers, transporting and handling these materials has been a problem. The problem is due to the agglomeration or sticking together of these polymers. These polymers are solid or semi-solid at room temperature. After polymerization, blending, or formulating, the polymers are solidified in some particulate form, such as pellets, spheres, granules or other shapes such as pillows. Those of ordinary skill in the art will appreciate that there are many geometries for such polymers. However, whatever geometry is chosen, agglomeration remains a problem.
There are several classes of polymers or polymer blends that are tacky or have a tendency to stick to one another or to their container. These may include, but are not limited to ethylene copolymers, propylene copolymers, or adhesives such as hot melt adhesives and pressure sensitive adhesives. With the growth of the volume of these materials produced in the world, the need to transport them in larger quantities has grown as well.
Generally, these polymers are rendered tacky by a number of factors. These factors include the comonomer amount, the comonomer type, the melt index or viscosity, the softness of the polymer, and the inherently tacky materials that can be added to polymers to improve their performance in intended end uses, such as adhesives. The low secant moduli and/or the relatively low softening points make the deformation of these materials under weight, pressure, and/or heat a particular problem. Such deformation allows the surface contact area of the particles, pellets, spheres, pillows or granules to increase. This deformation is known by several names such as creep or cold flow.
Further aggravating the polymer's tendency towards tackiness is the method of transport used to convey these products to the end user. Transporting modes can be 5 to 25 kilograms in a bag, as much as 300 or 1000 kilograms in boxes or bins, or even in bulk truck or rail containers where 15,000 to 90,000 kilograms are shipped at a time. In such shipments, the polymer's natural tendency to stick to itself or to the container is amplified by the weight of the polymer on itself, and, in the case of bags, the weight of other bags stacked one on the other for shipping and/or storage. Compounding this situation are warm or hot conditions such as seen in summer in much of the world, or year around conditions in tropical or semi-tropical climates. One reason for this aggravated tendency to agglomerate or stick together at elevated temperatures is that as the polymers get closer to or exceed their softening points they can deform to such an extent to stick together to form a large matrix or even a solid block.
However, in less severe conditions, the polymers may only stick together at their initial contact points without substantial deformation. Under such conditions the pellets or other polymer shapes may not fuse, but their intimate contact causes them to stick together, necessitating some form of physical breaking of the sticking to permit handling. Such physical breaking apart of agglomerated particles could be as simple as using a shovel or other instrument to weaken or break the sticking points such that the sticking points are eliminated to allow relatively free flow, or breaking enough of the sticking points to allow the material's weight to break the remaining sticking points. In either case this can cause use of additional labor, and lead to potential safety concerns. In some cases, freeing such fused polymer can be substantially impossible where cold flow has resulted in substantially a block of fused polymer or where the polymer is contained in a substantially inaccessible container.
Also contributing to the sticking, bridging, or fusing problem is the particle geometry and size. Smaller particles, in general, have greater surface area to volume ratios and, therefore, more contact than larger particles.
A number of ways to solve the agglomeration problem have been tried in the past with varying degrees of success. One method is to coat the materials with a microfine dust or powder of polyethylene homopolymer or copolymer. This method is generally not effective in cases where the powder has a higher melting point than the polymer coated, as the coating material can cause melting point variations, phase separation, poorer clarity (especially in adhesive formulations), loss of tenacity or bonding strength, or other problems in the fabrication or compounding of the coated polymers. This method is difficult to use commercially, as the powder or dust can cause explosion risks in a commercial environment. Additionally, there is usually only a weak bond between the coating and the base polymer, resulting in a partial removal of the coating during shipping and handling due to vibrations and particle movement. This removal will reduce or eliminate the coating's beneficial effect, and additionally pose another potential problem of loose dust that will complicate the handling of the polymer particles, for instance by plugging air filters on pneumatic conveying equipment.
U.S. Pat. No. 3,528,841 discloses a decrease in tackiness of polymers by coating with a parting agent that is a polyolefin powder. The powder is applied in the range of 250 ppm to 1000 ppm. The parting agent is dispersed in water by a block copolymer of ethylene oxide and propylene oxide.
U.S. Pat. No. 3,779,785 discloses a low melting point wax used as a parting agent for ethylene-vinyl acetate copolymers. The wax is employed in the form of a finely divided aqueous emulsion. The wax level is 1% to 5% by weight.
Another method of solving the sticking polymer problem is to blend into the polymer fatty amides that are known to be incompatible with the polymer. The incompatibility of these fatty amides will cause them to "bloom" or exude to the surface of the polymer particle. This method has been proposed to reduce sticking of pellets to one another. There are at least two problems associated with this proposed solution. The first is that when such additives are put into a polymer pellet, the blooming is a slow time related function. It is quite likely that these fatty amides will not bloom to the pellet surface in time to prevent agglomeration and/or fusing. Second, in many polymer applications, the polymer may not be processed through a melt extruder before loading in a container for transport, thereby eliminating the ability to add these additives without further cost and delay.
U.S. Pat. No. 4,510,281 discloses tack-free polymer pellets, defined by a stick point of 40.degree. C. at most (for ethylene vinyl acetate copolymers) and other polymers having a stick point of less than 60.degree. C. The polymers are ethylene vinyl acetates, terpolymers of ethylene with vinyl acetate and carbon monoxide, copolymers of methacrylic acid, copolymers of ethylene methylmethacrylate and terpolymers of ethylene, n-butyl acrylate and carbon monoxide. An ethylene bisoleamide additive is blended with the polymers and then melt extruded into pellets.
Another method to reduce the agglomeration problem is to change the shape or geometry of the solid polymer, polymer blend, or adhesive to, for instance, a "pillow" shape. Such a technique is viable usually only when the polymer or blend is adequately hard in the frozen or solid state at room or slightly elevated temperatures, to prevent sticking. Alternatively, reducing the contact area per unit volume, by making larger particles has practical limits for materials such as polyolefins which are usually pneumatically conveyed.
There exists a need for an easily applied, effective, non-deleterious coating and coating method that will permit storage, shipping and handling of many types of soft, tacky polymer particles.